[Test-Equipment] Hewlett-Packard 606A

[Richard Knoppow]

----- Original Message ----- 
From: "Richard Knoppow" <dickburk at ix.netcom.com>
To: "Discussion of Electronic Test Equipment" 
<test-equipment at mailman.qth.net>
Sent: Tuesday, June 17, 2008 6:52 PM
Subject: [Test-Equipment] Hewlett-Packard 606A


>    I recently obtained one of these guys at a local sale. 
> I worked for -hp- for many years and must have repaired 
> dozens of them but its been perhaps 40 years since I've 
> worked on one and I can't remember certain things.
>    This one has had plate bending done to the tuning 
> capacitor. In particular the plates at the high frequency 
> end have been bent to lower the capacitance. I can't 
> remember if this was a normal factory method. Usually 
> plate bending on any multiple band device is a no-no and I 
> suspect the plates should be straight on this one. Does 
> anyone know for certain?
>    All my notes were left behind when I left the company 
> although I remember some other instruments in great 
> detail.
>
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk at ix.netcom.com


     This is a report of work in progress.
     I was able to get the bent plates straight, or pretty 
close to it. The small end of the rotor is not very 
accessible but I managed it with the aid of a right angle 
needle nosed pliers.
     This is evidently the _correct_ condition. By 
alternately slipping the dial and adjusting the L and C trim 
I was able to get the dial into calibration all over on all 
bands. There slight errors but they are a fraction of the 
specification which is what I would expect. I have not yet 
checked the PA tuning mainly because my back protests after 
a certain amount of bending over work.
     The instructions specify a 428B clip-on milliampmeter. 
Its a very handy tool but I don't have one. The alternative 
is to unsolder the B+ lead to the amp and insert a 
conventional milliampmeter. However, there is a 100 Ohm 
resistor in series with the B+ and I suspect that a high 
resistance voltmeter will do the job since the voltage drop 
across this resistor (R-15 for those with the manual) should 
be proportional to the amplifier current. Since the 
capacitor plates for the amplifier were also bent I suspect 
it was tuned wrong as was the oscillator. I straightened 
these also.
     For those with similar instruments the dial is held in 
place by a clamp which is accessible when the main tuning 
knob is removed. The clamp is held in place by four screws. 
The dial can be slipped by loosening the screws. If you the 
frequency is off at center values when the end points are 
properly adjusted dial slipping will almost certainly get 
them on, provided the tuning cap has not been butchered. The 
procedure is simple but tedious. One can start by slipping 
the dial completely one way (so that, for instance, the low 
frequencies are under the fiducial) and the end points of 
one band set. See which way the middle frequencies are off. 
Then slip the dial all the way to the other side and again 
adjust the end points and see which way the middle is off. 
Then start with the dial about centered. Set the end points 
and re-check a couple of times to see that they are right. 
If the center is off you know which way to move the dial. 
Move it only a little. Then go through the whole routine 
again. If the center is better but still not on move the 
dial more the way you did the first time. If its off more 
then move the dial the other way. Eventually you will find 
the center frequencies will be either right on or very 
close.
     The specification is 1.0% but typically the accuracy 
should be much better than this, perhaps 0.25% or even 
better.
     More when I do the amp tuning (if it turns out to need 
it).
     In general older -hp- instruments should have errors no 
greater than half the spec. There are some exceptions, 
notably early 400D and H meters. These were poorly designed 
and there were at least three different versions. Late 
meters are much better than spec. but the early ones need 
considerable modification to even meet them.

More when I have more to report.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk at ix.netcom.com